The present invention relates generally to automated optical assembly and inspection machines.
These machines are used extensively in the field of surface mount technology (SMT). One example of such a machine is known as a pick and place machine. Pick and place machines are used to automatically place electronics components upon a circuit board during manufacture. Generally a component is picked up by a placement head that is movable with respect to the board. The placement head orients the component correctly and places the component at a desired location on the board.
In order to properly place components, it is necessary to determine the initial orientation and location of the component on the placement head. Additionally, the position of the placement head with respect to the board itself must be known. Once both are known, the position and orientation of the component are adjusted to place the component correctly upon the board. In order to sense component position and alignment, pick and place machines generally employ component align (CA) sensors. In order to sense the position of the placement head with respect to the board, such machines generally employ board align (BA) cameras. The BA camera is used to image a reference position marker or “fiducial” on the circuit board or board artwork. Analyzing the image of the fiducial allows calculation of the BA camera position with respect to the board. Since the BA camera is generally disposed on the placement head, for “on-head” machines, such analysis provides a relationship between the placement head location and the board.
As components get smaller, and placement accuracy increases, it is increasingly important to have very accurate component align sensors and board align cameras. One difficulty in providing accurate images is providing the correct illumination to the target to be imaged. Illumination can vary based upon the type of image being acquired, the reflectivity of the article, the reflectivity of the background of the article, and many other factors.
One popular method of illumination for board align cameras is known as co-axial illumination. Co-axial illumination is defined as illumination directed substantially along the line of sight of the imaging optics. Co-axial illumination is useful in that shadow images are minimized and surface features normal to the optical axis of the camera are illuminated efficiently, especially for objects with specular surfaces. Typically, co-axial illumination is effected using a light source projecting illumination from the side of the optical axis into a beamsplitter which bends the beam such that it aligns with the optical axis of the camera. This technique suffers from at least two limitations. First, the beamsplitter is not an economical part and thus can significantly increase the cost of the system. Second, the outgoing beam is attenuated by the beamsplitter and substantially decreases efficiency for diffuse targets.